1. Field of the Invention
The present invention relates to a method of producing zinc oxide powder.
2. Description
Zinc oxide is a consumption product frequently used in the ceramics, industrial rubber and tire industries, in the chemical and pharmaceutical fields and in the production of food supplements.
The method most frequently used for producing zinc oxide is the so-called “French” or “indirect” process, during which the oxide is produced from metallic zinc, by evaporation thereof in a crucible or in a distillation column, and then by oxidation of the fumes. According to the purity of the starting zinc, it is possible to obtain a more or less pure zinc oxide, in the form of fine particles having a nodular structure and a mean size (d50) of between 0.5 and 5 μm. These particles develop a relatively low specific surface, calculated according to the BET method, of around 3 to 10 m2/g.
It is also possible to product zinc oxide by the so-called “American” or “direct” process. This method also produces the oxide from a volatilisation of zinc, followed by an oxidation of the fumes. However, this method makes it possible to process raw materials other than previously refined metallic zinc, by adding a reducing compound in the feed of the furnace used. The oxides obtained have a particle size and specific surface substantially similar to those obtained by the “French” process. These particles do however have an acicular structure that makes them unsuitable for use in several fields, in particular in the tire and rubber industry. The chemical purity of the powder obtained is also low, given the diversity of the nature of the raw materials processed.
Finally, hydrometallurgical processes are known (see for example W J Wendt, Ammonia, Ammonium Carbonate Leaching of Low Grade Zinc Ores, in Engineering and Mining Journal, 1953, vol. 154, pages 84-90; WO-98/36102, U.S. Pat. No. 2,147,379). These methods consist of drying and/or calcining a zinc hydroxide or zinc carbonate obtained by the precipitation of solutions of zinc salts (chloride, sulphate, hydrosulphite, ammoniacal complexes, etc). These methods are generally employed to reprocess by-products from the chemical industry or for processing zinc ore. According to various parameters, the zinc oxides produced in this way have relatively different properties and are often sold as zinc concentrates. The specific surfaces are very variable, from 3 to 80 m2/g, with mean particle sizes that are difficult to control and may range up to more 100 μm. Despite all the purification steps designed up till now, the zinc oxides produced have undesirable chloride and sulphate contents. The application of these materials is therefore possible only sporadically or in certain niche sectors.